1. Field of the Invention
This invention relates to an improved coal injection and coal feed system for use with a coal-fired firetube boiler.
2. Description of the Prior Art
Currently there is a very large number of gas-fired boilers which are operational. In a typical gas-fired boiler, the fuel combustion takes place in a firetube with the walls of the tube being heated by the combustion. Water is circulated past the outer wall of the tube and in heat transfer relationship to the walls of the firetube, so that the water is converted to steam. In a typical boiler, the heated gases from the combustion are caused to flow along several additional tubes which are contained within the boiler, with the external walls of these additional tubes being also exposed to the water so as to increase the efficiency of heat transfer from the hot combustion gases to the water and thereby increase the efficiency of the steam-formation function.
Gas-fired boilers commonly are fueled by means of natural gas, propane or other gaseous fuel, or by oil (which is mixed with air to generate a type of mist that is injected into the firetube). In Public Law 99-190, Laws of the 99th Congress-1st Session, it was mandated "to rehabilitate and convert current steam-generating plants at defense facilities in the U.S. to coal-burning facilities in order to achieve a coal consumption target of 1,600,000 short tons of coal per year above current consumption levels at Department of Defense facilities in the United States by fiscal year 1994; Provided, That anthracite or bituminous coal shall be the source of energy at such installations; Provided further, That during the implementation of this proposal, the amount of anthracite coal purchased by the Department shall remain at least at the current annual purchase level, 302,000 short tons."Successful completion of this mandate, at minimum cost, dictates that there be a conversion of the existing gas-fired boilers to coal-fired boilers.
Conversion of a firetube boiler to a coal-fired boiler is complicated by reason of the relatively short length of the firetube. Combustion of a gas or oil fuel in a boiler requires less lineal distance for the combustion reaction than for the combustion of coal as the fuel. This is due in major part to the fact that conversion of the carbon content of the coal requires a longer time period than does the conversion of the carbon content of the gas or oil fuels. Consequently, firetube boilers have a smaller combustion volume than coal-fired boilers. Further, in firetube boilers, there is a high rate of heat loss to the water-cooled walls of the tubes within the boiler, which rate of heat loss adversely affects the combustion rate of coal burned in the same firetube.
Goals for coal-fired boilers include (1) greater than 99% carbon conversion efficiency, (2) greater than 80% boiler efficiency, (3) NO.sub.x emission less than 0.7 lb/MBtu, and (4) turndown ratio of 3-to-1.
Coal delivery systems are used in conjunction with coal-fired boilers to deliver coal to the boiler. Prior art coal delivery systems have used large pressurized coal storage tanks in conjunction with an airlock system to deliver coal to the boiler. Such prior art coal delivery systems have also required the use of a control valve at the coal feed line in order to regulate the flow of coal to the boiler. Such control valves can create a restriction in the flow area which is a source of plugging when micronized material, such as finely divided coal, is injected through the feed line into the boiler.